The photographic industry is engaged in a continual effort to improve the stability of its products. Stability is typically used as a "catch-all" phrase for the resistance of a given photographic element to changes in its photographic properties. In fact, stability actually embodies at least three distinct characteristics of a photographic element, with each characteristic having a unique impact on the element's overall quality.
Stability can take the form of raw stock stability, latent image stability, or image stability. As will be discussed in subsequent paragraphs, each form of stability is due to a unique interaction between the components of a photographic element. Thus, compounds and processes that are capable of being utilized to improve one aspect of stability will not necessarily, and often do not, improve other aspects of stability.
It is known that certain classes of compounds are useful for stabilizing the developed visible image that is obtained during the processing of an exposed photographic element. In black and white photography, the visible image is comprised of filamentary silver. In order to preserve the image, it is important that the filamentary silver be stabilized against changes in storage conditions, and against interaction with other components of the element or with components left in the element from processing solutions. This is typically accomplished by compounds which strongly adsorb to the filamentary silver and passivate its surfaces.
British Patent 1,186,441 describes certain heterocyclic mercapto compounds for stabilizing developed photographic silver images. Similarly, German Patent 1,522,363 describes the use of specific mercapto compounds for stabilizing a developed silver image.
In color photography, a visible image is formed which is comprised of dyes instead of metallic silver. Such dyes may, over time, deteriorate due to their interaction with UV radiation, atmospheric oxygen, or other elements of the environment. In order to obtain an image that is stable over time, certain dye stabilizers can be added to the photographic element. For example, U.S. Pat. Nos. 3,432,300, 3,574,627, 3,764,337, and 4,042,394 describe dye stabilizers capable of being incorporated into a photographic element. In the photothermographic art, which requires dry processing at elevated temperatures to provide a visible dye image, it is known to incorporate certain alkoxycarbonyl mercaptans to improve dye stability after processing. U.S. Pat. Nos. 3,839,041 and 5,084,376 describe such elements and compounds. U.S. Pat. No. 4,689,286 describes a similar photothermographic element containing alkyl or aryl substituted thiocarbonic acid esters for fog suppression.
When conventional silver halide photographic elements are exposed to actinic radiation, a record of the exposure invisible to the unaided eye is formed. This invisible record of exposure is referred to as a latent image; and it is generally agreed that the latent image comprises minute specks of metallic silver formed in, or on, individual silver halide grains. Formation of the latent image is believed to be due to the interaction of silver ions with photoelectrons generated by the absorption of actinic radiation by silver halide grains.
It is known that the presence of a latent image is necessary for the development of a visible image. It is also known that the latent image is not permanent and that, with the passage of time, can be lost, thus converting silver halide grains which would be developable immediately after exposure into nondevelopable silver halide grains. The phenomenon of losing a latent image is termed latent image fading, and it manifests itself as a loss in image density in the developed image, and a consequent loss in sensitivity in the photographic element.
Latent image fading can present a significant problem to film builders. However, it can be eliminated or substantially reduced by application of known latent image stabilizers, which function by a mechanism that is presently not completely understood. It is believed that different kinds of latent image stabilizers may function by different mechanisms. One class of latent image stabilizers is the mercaptotriazoles described in U.S. Pat. No. 5,089,381. EP 0 377 889 describes a class of triazolomercaptan latent image stabilizers.
Stabilization also embodies raw stock stabilization, often referred to as storage stability or raw stock keeping (RSK). This form of stabilization typically manifests itself in a photographic element's resistance to fog formation or sensitivity change during prolonged storage, particularly during prolonged storage under conditions of high temperature and relative humidity. In recent years, because of the increased use in the photographic industry of silver chloride emulsions, which exhibit a greater propensity for storage deterioration than silver bromide or silver iodobromide emulsions, considerable effort has gone into finding effective raw stock stabilizers.
Attempts have been made to improve raw stock stabilization by the addition of inhibitory agents to the silver halide emulsions. These fog inhibiting agents, however, have often proved inadequate. Examples of raw stock stabilizers are described in U.S. Pat. Nos. 2,772,164, 2,819,965, 2,897,081, 2,919,985, 2,952,539, 2,981,624, 3,051,570, GB 858,326, and JP-A-094626. The compounds in these references generally comprise heterocyclic carboxy- or alkoxycarbonyl-alkyl mercapto structures.
Still other forms of stabilizers are known in the art. U.S. Pat. No. 3,791,830, for example, describes the use of arylmercaptoethyl or arylsulfonylethyl esters of carbonthioic acids as antifoggant precursors for stabilizing a photographic element against overdevelopment. U.S. Pat. No. 4,396,707 describes the use of certain aminotriazolomercapto compounds for fog control when processing silver halide photographic element at elevated temperatures.
Other alkoxycarbonylmercapto compounds are described in U.S. Pat. No. 5,081,009 and JP 63-046458 as alkali cleavable precursors to mercapto compounds in reversal reflective printing materials, or direct positive internal latent image silver halide emulsions. In U.S. Pat. No. 4,522,917, a photographic element is described which contains a compound capable of undergoing alkali hydrolysis during development to release a photographically useful group comprising an amino moiety. In U.S. Pat. No. 4,952,491, mercaptoazoles or their precursors are described for use in tabular grain emulsions comprising at least about 50 mol % of silver chloride. These compounds, however, have been found to cause a substantial loss in emulsion sensitivity.
Despite the myriad forms of stabilizers known in the art, there has yet been provided a sufficiently effective class of stabilizers that are particularly suited for the raw stock stabilization of color negative or reversal photographic elements.